1. Field of the Invention
The invention relates generally to a method and device for detecting analytes in ambient air and specifically to a method and device for accelerating chemical sensor kinetics and for restoring a baseline for a chemical sensor.
2. Description of the Related Art
Ambient air sampling for chemical sensor applications can take a variety of forms that can broadly be separated into two classes, passive and active sampling. In passive sampling, the ambient gas is exposed to the sensor without active pumping from a pump that is integral to the sensor or detector. The sensor can be exposed directly to the ambient air with or without a membrane that serves to prevent contamination of the sensor. Signal kinetics are relatively slow and are largely controlled by diffusion and the thickness of the polymer coated sensor. No accommodation for signal baseline restoration is incorporated in a passive sampling mode, and if the device suffers from baseline drift due to for example humidity or temperature changes, the signal contribution from an analyte exposure will become uncertain under dynamically changing environmental conditions. Active sampling allows active flow of the air sample and can direct the flow through pneumatic tubing, valve(s), collection devices, and gas chromatographic columns prior to or down stream of the sensor. Active sampling draws sample past the polymer coated sensor, but does little to accelerate the sorption or desorption of the analyte molecules from the active surface of a chemical sensor.
Accordingly, it is an object of the present invention to provide a method and apparatus for delivering a sample of ambient air/gas to a chemical sensor resulting in accelerated signal kinetics.
It is a further object of the present invention to provide a method and apparatus for delivering a sample of ambient air to a chemical sensor wherein the interaction of molecules of an analyte in the gas sample with the surface of the sensor is maximized.
It is a further object of the present invention to provide a method and apparatus for delivering a sample of ambient air to a chemical sensor whereby the interaction of interfering molecules, such as water in the gas sample, with the surface of the chemical sensor can be minimized.
It is a further object of the present invention to provide a method and apparatus for delivering samples of ambient air to a chemical sensor whereby a baseline for the chemical sensor can be established between discrete samples.
It is a further object of the present invention to provide a pulsed gas sampling technique that increases the sorption rate of analyte into a sorbent coated chemical sensor by up to 3 orders of magnitude faster than conventional pneumatic gas sampling techniques that pump air over a sorbent film at atmospheric pressure.
It is a further object of the present invention to provide a pulsed gas sampling technique that allows rapid clean up of a sorbent coated sensor to a vapor-free state that allows the identification of an essentially analyte-free baseline.
These and other objects are attained by an apparatus for detecting the presence of at least one analyte in ambient air, the apparatus comprising
at least one sensing element comprising a surface having at least one sorbent coating, such as a chemoselective polymer coating, that selectively interacts with at least one analyte and provides a detector for detecting the selective interaction of the at least one chemoselective polymer with the analyte,
a housing that encloses an environment surrounding the at least one sensing element, the housing including an inlet port connected to a sampling pump for taking a gaseous sample from ambient air into the housing under reduced pressure, a valve or something similar for sealing the environment surrounding the sensing element so that the environment surrounding the at least one sensing element can be isolated from ambient air and evacuated, and an outlet port connected to the sampling pump under vacuum for removing the gaseous sample from the environment surrounding the at least one sensing element,
wherein the size and orientation of the inlet port are selected so that the gaseous sample is directed to strike each at least one sensing element in a turbulent flow that is substantially perpendicular to the surface of the sensing element having the chemoselective polymer.
These and other objects are further attained by a method of monitoring ambient air to detect the presence of an analyte, the method comprising the steps of
(a) providing an apparatus that comprises a sensing element comprising a surface having a sorbent polymer coating that selectively interacts with the analyte and means to detect the selective interaction of the sorbent coating with the analyte, a housing that encloses an environment surrounding the sensing element, the housing including an inlet port connected to a sampling pump for removing or collecting a gaseous sample from ambient air and taking the gaseous sample into the housing under pressure, wherein the size and orientation of the inlet port are selected so that the gaseous sample is directed to the sensing element in a turbulent flow that is substantially perpendicular to the surface having the sorbent coating, an outlet port for removing the gaseous sample from the environment surrounding the sensing element and means for sealing the environment surrounding the sensing element so that the environment surrounding the sensing element can be isolated from ambient air and evacuated,
(b) sealing the environment surrounding the sensing element from ambient air and evacuating the environment, so that a baseline for the sensing element is established,
(c) collecting a gaseous sample from ambient air and taking the gaseous sample into the housing initially under reduced pressure so that the gaseous sample is directed to strike the sensing element in a turbulent flow that is substantially perpendicular to the surface having the chemoselective polymer coating, whereby molecules of the analyte, if present in the gaseous sample, interact with the chemoselective polymer and wherein any such interaction is detected, and whereby low molecular weight species not of interest in the gaseous sample are propelled by the turbulent flow towards the outlet port, and (d) repeating steps (b)-(c) to cyclically monitor the ambient gas for the presence of the analyte and restore the sensing element to its baseline.